The present invention relates to a ringing control circuit in a telephone switching system, in which a ringing signal is automatically stopped upon detection of an answer signal, and the switching operation is performed.
FIG. 1 shows a prior ringing control circuit. That kind of circuit is shown in U.S. Pat. No. 3,978,293, and U.S. Pat. No. 3,076,059, in which a ringing loop current detector which detects an answer signal is shared by groups of the line circuits for detecting call answer signals during active ringing on a call and for controlling the line circuits to stop the ringing upon a called party answer.
In FIG. 1, the symbol A is a ring trip circuit, B.sub.0 through B.sub.3 are subscriber ringing controllers, C.sub.0 through C.sub.3 and subscriber line scan circuits, SUB.sub.0 through SUB.sub.3 are subscribers, LC.sub.0 through LC.sub.3 are subscriber line circuits, (a) is an answer detection signal which shows that an answer signal is detected, b.sub.0 through b.sub.3 are subscriber ringing set signals, c.sub.0 through c.sub.3 are subscriber line scan signals.
In FIG. 1, when the first subscriber SUB.sub.0 is called, the ringing set signal b.sub.0 is originated in a computerized central control unit (not shown), and said signal b.sub.0 is applied to the first ringing controller B.sub.0, to which a continuous ringing signal CR is supplied from the ringing source RS. The ringing controller B.sub.0 interrupts the continuous ringing signal CR to supply a ringing signal IR to the subscriber SUB.sub.0. The interruption of the continuous ringing signal CR, or the period of the ringing signal IR is designated by the ringing control signal b.sub.0. Thus, the subscriber SUB.sub.0 is called by the ringing signal IR.
When other subscribers SUB.sub.1 and/or SUB.sub.2 are called at the same time as the first subscriber SUB.sub.0 is being called, those subscribers are called simultaneously. In that case, the phase for the first ringing signal IR.sub.1 to the first subscriber is different from that of other subscribers. That is to say, the active ringing interval and the silent ringing interval are staggered for each of the subscribers. Those staggered ringing signals are called the first phase ringing signal (I), the second phase ringing signal (II), and the third phase ringing signal (III), etc. (see FIG. 2). When the continuous ringing signal CR is divided to three phase ringing signals, the fourth subscriber can not be called, and if a call arrives to the fourth subscriber during the time these three subscribers are being called or ringing, the fourth subscriber is recognized as a busy subscriber. When the first subscriber SUB.sub.0 lifts a handset in an active status, the loop current in the first subscriber is interrupted, and the ring trip circuit A detects that situation and provides the answer detection signal (a), which is supplied to all the subscriber ringing controllers B.sub.0 through B.sub.3. Then, all the ringing controllers B.sub.0 through B.sub.3 are released to stop the ringing operation of the subscribers. Thus, the answer signal of a single subscriber releases all the subscribers (see t.sub.1 of FIG. 2). Next, the central control unit (not shown) scans the subscriber circuits LC.sub.0 through LC.sub.3 through the subscriber line scan signals c.sub.0 through c.sub.3, to detect which subscriber has not answered yet. If the subscribers SUB.sub.1 and SUB.sub.2 have not answered yet, the central control unit supplies the second phase control signal b.sub.1 to the ringing controller B.sub.1, and the third phase ringing control signal b.sub.2 to the ringing controller B.sub.2. Thus, the subscribers SUB.sub.1 and SUB.sub.2 are called or rung again at time t.sub.2 (see FIG. 2).
Accordingly, when a single subscriber answers, the ringing signals of all the subscribers belonging to that subscriber group are stopped, and the ringing signals for the subscribers which have not answered yet are initiated again under the control of the central control unit. As the central control unit is generally implemented by a programmed computer, the structure of the program must be very complicated to handle those ringing signals. Further, when the program of the central control unit is in the queue, the program for ringing a subscriber must have a high priority for providing a ringing signal to a subscriber as if a ringing signal was not interrupted.
That interruption of a ringing signal of a subscriber which has not answered, and/or the high priority process of a ringing signal in the central control unit restrict the process capability of the central control unit, or restrict the number of subscribers who can belong to a single central control unit.